The present invention relates to ceramic cutting tool materials and, in particular, to such cutting materials in which monocrystalline whiskers are uniformly distributed in a ceramic matrix containing aluminum oxide which leads to an improved strength and toughness and, in particular, improved thermal shock resistance without negatively influencing the wear resistance.
Ceramic cutting tools have now been available for several decades, but they have not until the last years, had any great commercial importance for use in chipforming machining. The main reason for the limited growth of ceramic cutting tools has been sudden and unexpected tool failures because of their inherent inadequate strength, toughness and thermal shock resistance.
In the last decade, the properties of ceramic cutting tool materials have been improved in many respects and their use in cutting of cast iron and aerospace alloys have increased. In the cutting of steel, which is the completely dominating work piece material, the use of ceramic cutting inserts is still very modest because said workpiece material makes large simultaneous demands upon strength, toughness and wear and thermal shock resistance which have only partially been fulfilled by currently known ceramic cutting tool materials.
Aluminum oxide-based cutting tool materials are very sensitive to thermal crack formation because aluminum oxide in itself, has a relatively low thermal conductivity. This leads to very short tool lives in steel machining, particularly under conditions with short engagement times and varying cutting depths.
To a certain extent, the thermal properties have been improved by additions of titanium carbide and/or titanium nitride which enhance the thermal conductivity of the composite material. The addition of titanium carbide/nitride also increases the hardness of the material. In comparison with pure aluminum oxide materials, an increased tool life is therefore obtained in the cutting of harder work piece materials and in operations demanding resistance to thermal shocks. However, this kind of material has too poor a toughness behavior for general use in the cutting of steel.
Another later step of development relates to uniformly dispersed, fine-grained zirconium oxide particles in a matrix of aluminum oxide. A transition of the tetragonal `metastable` zirconium oxide particles to a stable monoclinic phase with a concomitant increase in the volume of the ZrO.sub.2 particles during use increases both strength and toughness and thus leads to a more predicable tool life.
The thermal properties of said type of materials are however, only slightly better than those of pure aluminum oxide materials. Therefore, initiation and growth of thermally induced cracks is still a great problem in practical cutting operations generating high cutting edge temperature such as the cutting of steel.
It has been shown that reinforcement with SiC whiskers of a matrix of aluminum oxide leads to a greatly improved fracture toughness and strength. Ceramic cutting tool materials based upon said concept have shown very good performance in the cutting of hot strength materials in particular, but in the cutting of steel they have shown surprisingly short tool lives because of preferential chemical attach of the SiC whiskers. This leads to a weakening of the surface zone with accompanying high wear and risks of crack initiation.
In U.S. Pat. No. 4,867,761, oxide-based ceramic cutting tool materials are strengthened by whiskers of carbides, nitrides and borides of Ti or Zr or solid solutions thereof, having a low solubility in steel resulting in a cutting tool material having at the same time an improved, more predictable toughness, as well as improved strength and resistance to thermal shocks without deterioration of the wear resistance to any appreciable degree, particularly when cutting steel. This was not possible with earlier known material compositions.
Further improvements, especially with respect to thermal shock resistance and fracture toughness were made in U.S. Pat. Nos. 5,141,901 and 5,231,060, disclosing oxide-based ceramic cutting tool materials strengthened by whiskers having a linear thermal expansion coefficient being lower than that of Al.sub.2 O.sub.3, preferably at the most 85% of that for Al.sub.2 O.sub.3 measured at 300-1300K. The preferred whiskers being carbides, nitrides and borides of Ta, carbides of Nb, Hf and V and nitrides of Hf.
Swedish Patent Application 9601335-4, which corresponds to U.S. Pat. No. 5,851,285, herein incorporated by reference, discloses a method of producing whiskers in large volumes and at low cost to be used as reinforcing material. The whiskers consist of solid solutions between two or more transition metal carbides, nitrides and carbonitrides having preferably submicron diameters.
The properties of a whisker reinforced cutting tool material based on alumina, depend to a large extent on the properties of the whiskers themselves and also on the difference in thermoelastic properties between the whisker and the matrix material. It is known that wear resistance when machining steel depends on the solubility of the whisker material in steel since alumina is practically insoluble. Experimentally, it has been found that the crater wear rates of different carbide coatings when machining steel can be reasonably predicted from solubility data. Experimentally, the following relative crater wear rates have been found.
TABLE 1 ______________________________________ Relative Crater Wear Rates When Machining Steel Carbide Wear Rate ______________________________________ HfC 1 TiC 2.6 ZrC 5.7 TaC 10.4 NbC 16.7 ______________________________________
The thermal shock and fracture toughness properties of the composite depend on the thermal expansion coefficient for the whisker material. The corresponding thermal expansion coefficients are given in Table 2.
TABLE 2 ______________________________________ Thermal Expansion Coefficient (300-1300.degree. C.) Thermal Expansion Coefficient Carbide 10.sup.-6 .multidot. K.sup.-1 ______________________________________ HfC 6.5 TiC 7.5 ZrC 7.0 TaC 4-6.5 NbC 5-7 ______________________________________
When comparing Tables 1 and 2, it is obvious that whiskers with the highest potential for a thermal shock resistant composite, e.g., TaC, have a considerably lower wear resistance than TiC, which might give a composite prone to thermal cracking under heavy thermal loads.
Alumina based composites made from carbide, nitride and boride whiskers of transition metals such as Ti, Ta, Nb, Zr and Hf, according to the prior art, consequently have a rather narrow application range since the composite properties depend mainly on the properties of the whisker itself, which are given by the transition metal in question. This means that for many applications, the optimum properties cannot be arrived at using any of the aforementioned whiskers.